I went from an i7 to a C2Q with a 560ti, and now almost every game is unplayable even at lower settings (BF3, ARMA2, just about anything that isn't GPU dependent.) So he won't see much if any of an upgrade.

Actually I agree (never thought about it). My x3 710 is comparable to a C2D and even paired with my GTX 460 I run into CPU limitations in game (so a 570 definitely would). It ranges from time to time CPU limitations in some games to always in others (games like Civ V and SCII late game). I say hang on to the 5850 and save the cash for a CPU/MOB/Ram upgrade.

Link to comparison of x3 710 to C2D e7500 who don't think they are comparable (they are very close as I was deciding between the two when I purchased).AnandTech - Bench - CPU

My next upgrade will be CPU/MOB/Ram for sure (though who knows when that will be).